A variety of applications exist requiring accurate and sensitive detection of fluid leakage or determination of fluid levels. For instance, the transportation of fluids such as liquid chemicals, various oils including crude oil, gasoline, petroleum, and other liquid organic solvents, is routinely performed via underground pipelines. Early detection of leaks in such fluid transport systems is critical in view of a number of factors including the dangers of explosion, corrosion and environmental pollution arising due to leakage of the fluid being transported. The situation is complicated because thepipe lines are often located in places not easily accessible and are normally covered with heavy thermally insulating and corrosion resistant layers.
A fairly conventional approach to this problem is the use of leakage detection cables in systems which are based upon the transmission of pulse signals over a pair of coaxially-arranged conductors and measurement of any variation in the characteristic impedance between the conductors in order to determine the presence of a fluid. More specifically, in such systems detection of the presence of a fluid is accomplished by using a coaxial cable having a porous outer conducting layer and a dielectric insulation layer laid out in such a way that fluid permeating the outer conductor fills air spaces within the dielectric material and thereby causes the dielectric permittivity as well as the characteristic impedance at a particular location to change. In coaxial cable-based fluid detection systems, the cable is energized by a pulse signal generator and the appearance along the cable of either reflected or standing waves arising from deviation of the characteristic impedance of the cable from its initial value is recorded. The extent of reflection or absorption of pulse signals so measured is then used to determine fluid level. Such coaxial detection cables permit remote determination of the presence of fluid in the vicinity of the cable. This technique of measurement is well known in the field of fluid detection and will not be discussed here in detail.
Conventional fluid detection cables are generally composed of a pair of coaxial conductors separated from each other by an insulation material which is partially porous to the liquid being detected and permits retention of fluid within air gaps defined in the material to cause the electrical characteristics between the coaxial conductors to change measurably in presence of the fluid. Typically, coaxial detecting cables are constructed with a solid inner conductor coaxially surrounded by a hollow braided outer conductor with the space between the conductors being filled by a porous dielectric layer such as fiberglass yarn. The presence of fluid is detected by the change in cable impedance when the air spaces in the dielectric yarn are filled with the fluid. A common problem with such cables is that the impedance change occurs at a slow pace because of the time required for the fluid to permeate through the outer conductor and settle into the air spaces defined within the fiberglass yarn. In addition, once a cable has been immersed in the fluid being detected and the resultant change in impedance recorded, drying the dielectric material, i.e., getting rid of the fluid that has accumulated in the air spaces within the dielectric, is quite problematic and involves substantial effort and time in returning the changed characteristic impedance of the cable back to its initial value before proceeding with successive fluid detection measurements.